The disclosure of Japanese Patent Application No. 2001-126600 filed on Apr. 24, 2001 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to an apparatus for controlling the temperature of a fluid in a power-transmission system, and to a method of controlling the temperature of the fluid.
2. Description of Related Art
In one of known power trains, a transmission is provided via a hydrodynamic power-transmission system on the output side of a vehicular engine. This hydrodynamic power-transmission system is designed to transmit power between one rotational member and the other rotational member by means of the kinetic energy of hydraulic fluid. An oil passage for hydraulic fluid used in the hydrodynamic power-transmission system is connected to an oil passage on the side of the transmission. The same hydraulic fluid is used in the hydrodynamic power-transmission system and the transmission.
Even if torque fluctuations occur in a driving-power source in the hydrodynamic power-transmission system, they are absorbed or alleviated due to the slippage occurring between one rotational member and the other rotational member. However, hydraulic fluid possesses the property of increasing in viscosity in proportion to a decrease in temperature. Therefore, the shearing resistance of hydraulic fluid resulting from a difference between a speed of one rotational member and a speed of the other rotational speed increases in the surroundings of low outside air temperatures. Thus, the amount of power loss in the driving-power source resulting from the shearing resistance of hydraulic fluid, namely, the amount of power loss resulting from a dragging torque increases. This constitutes the cause of a deterioration in fuel consumption.
Japanese Patent Laid-Open Publication No. 10-77834 discloses an engine cooling system capable of solving such a problem. The system disclosed in this publication is designed such that coolant for a water-cooled engine circulates through a coolant circuit with the aid of a water pump. The engine drives the water pump. The coolant circuit extends through the engine. A heat-accumulating tank for thermally insulating and storing coolant that has circulated through the engine is provided. In addition, a heat exchanger is disposed downstream of the heat-accumulating tank in the direction in which coolant flows through the coolant circuit.
The aforementioned cooling system is designed such that the engine is cooled through transmission of heat from the engine to coolant and the coolant that has been heated through transmission of heat from the engine is stored in the heat-accumulating tank.
In addition, coolant in the heat-accumulating tank warms engine oil and hydraulic fluid for the hydrodynamic power-transmission system if the hydraulic fluid is at a low temperature, for example, immediately after the engine has been started. As a result, the viscosity of engine oil for lubricating moving parts of the engine and the viscosity of hydraulic fluid for the hydrodynamic power-transmission system decreases, whereby it becomes possible to reduce frictional loss and thus achieve an improvement in fuel consumption.
However, since the cooling system disclosed in the aforementioned publication requires a heat-accumulating tank, the number of parts required for the system is great. This leads to a problem of structural complication of the system as well as an increase in weight, size, and cost of the system.
It is an objective of the invention to provide a device capable of controlling the temperature of fluid for a power-transmission system without increasing the number of parts required for the device.
A first aspect of the invention relates to a control apparatus capable of controlling a temperature of fluid. The control apparatus includes a power-transmission system. The control apparatus is provided with temperature controller that controls a temperature of fluid supplied to the power-transmission system by controlling a sub-system associated with functions of the power-transmission system while a vehicle is coasting. The fluid includes hydraulic fluid in a hydrodynamic power-transmission system and lubricant in the transmission.
In the first aspect of the invention, since the temperature of fluid is adjusted by controlling the sub-system while the vehicle is coasting, the temperature of fluid can be controlled without the necessity of providing a specially designed heat-accumulating device. Accordingly, there is no need to increase the number of parts in addition to the original system. Thus, it is possible to prevent structural complication of the system and suppress an increase in weight, size, and cost of the system. In addition, since the sub-system associated with the functions of the power-transmission system is controlled when the vehicle is coasting, the acceleration performance of the vehicle remains unaffected.
A second aspect of the invention relates to a method of controlling a temperature of fluid for a power-transmission system. This method comprises a first step of supplying the fluid to the power-transmission system, a second step of determining whether or not a vehicle is coasting, and a third step of controlling a sub-system associated with functions of the power-transmission system so as to control a temperature of the fluid if it is determined that the vehicle is coasting.
In the second aspect of the invention, the sub-system associated with the functions of the power-transmission system is controlled in the third step so as to adjust a temperature of fluid while the vehicle is coasting. Therefore, the temperature of fluid can be controlled without the necessity of providing a specially designed heat-accumulating device. Accordingly, there is no need to increase the number of parts in addition to the original system. Thus, it is possible to prevent structural complication of the device and suppress an increase in weight, size, or cost of the device. In addition, since the sub-system associated with the functions of the power-transmission system is controlled when the vehicle is coasting, the acceleration performance of the vehicle remains unaffected.